The present invention relates to a coupling-extracting device for a signal transmitted by optical fibres. It is more particularly intended to be inserted in a transmission optical fibre so as to make it possible to extract, whilst maintaining the continuity of transmission of the initial signal through the device from upstream to downstream, part of the signal to a detector, and at the same time to re-introduce a new signal in a downstream direction; moreover, it is possible for this new signal to be either the same extracted signal, after regeneration, or another signal, carrying new information.